Electrochemical cell

ABSTRACT

An electrochemical cell having two or more diffusion bonded layers, which demonstrates a high degree of ruggedness, reliability, efficiency and attitude insensitiveness, is provided. The novel cell structure simplifies construction and operation of these cells. Also provided is a method for passive water removal from these cells. The inventive cell, as well as stacks made using these cells, is suitable for use in applications such as commercial space power systems, long endurance aircraft, undersea power systems, remote backup power systems, and regenerative fuel cells.

TECHNICAL FIELD

The present invention generally relates to an improved electrochemicalcell, and more particularly relates to an electrochemical cell havingtwo or more diffusion bonded layers.

BACKGROUND AND SUMMARY OF THE INVENTION

Electrochemical cell devices are typically made up of a plurality ofelectrochemical cells, arranged in groups or stacks, and commonly serveto electrolytically disassociate water or another liquid (with orwithout dissolved constituents) into its components (i.e., electrolysiscells), or catalytically combine hydrogen or other fuel and an oxidizer(i.e., fuel cells), with electricity being either supplied or generated,respectively. Other related functions for electrochemical cell devicesinclude their use as compressors, separation and/or purification means,sensors, and combinations of these functions.

Within arranged groups or stacks, each electrochemical cell includes acathode, an electrolyte (e.g., a membrane), and an anode. In ProtonExchange Membrane or PEM cells, where the electrolyte is a cationexchange membrane, the cathode/membrane/anode assembly (i.e., “membraneelectrode assembly” or “MEA”) is typically supported on both sides byflow fields made up of screen packs or channeled plates. Flow fields,usually in the form of expanded metal or woven screens, oradhesive-bonded, laminated, or machined assemblies, facilitate fluidmovement, removal of product water, and also serve to provide in, forexample, PEM cells, mechanical support for the MEA.

By way of example, U.S. Pat. No. 5,316,644 to Titterington of al.teaches an electrochemical cell electrode plate structure that comprisesa laminar assembly of at least two substantially identically configuredand etched plate-shaped components. The plate-shaped components areadhered or bonded together using so-called laminating substances such asvarious epoxy resins, silicon and FLUOREL® elastomers and TEFLON®fluoroethylene propylene or FEP copolymers (see col. 9, lines 11 to 16,of U.S. Pat. No. 5,316,644). Cells made using these laminated electrodeplate structures are known to have a high degree of flatness andstrength. The process used to build these plate structures, however, isboth time consuming and difficult to control. Moreover, plate structuresthat are built using this process are comprised of distinct layers thatmay degrade or exhibit high resistance at the interfaces.

A need exists for an electrochemical cell that overcomes the drawbacksassociated with cells made using adhesive-bonded or laminated platestructures.

The present invention satisfies this need by providing anelectrochemical cell that comprises two or more diffusion bonded layers,the diffusion bonded layers demonstrating excellent conductivity andimproved resistance to delamination.

In a preferred embodiment, the inventive electrochemical cell comprisesa diffusion bonded laminar or thin plate assembly in the form of, forexample, a partially or fully diffusion bonded bipolar plate assembly.

The present invention further provides an arranged group or stack of theabove-described electrochemical cells, with each such electrochemicalcell preferably comprising either a partially or fully diffusion bondedbipolar plate assembly.

Each electrochemical cell in the stack will typically employ a porousplate/frame assembly for water/gas separation. Such an assembly mayutilize a metallic or polymeric porous membrane. For metallic porousmembranes (e.g., sintered metallic porous membranes), the membrane maybe directly diffusion bonded into a bipolar plate assembly. Forpolymeric porous membranes, the membrane is preferably incorporated intothe cell as a separate item. For example, the polymeric porous membranewould be supported by a first diffusion bonded plate assembly (e.g.,oxygen screen/frame assembly) on one side, and a second diffusion bondedplate assembly (e.g. water chamber//divider sheet//coolantchamber//divider sheet//hydrogen chamber) on the other side.

In a first more preferred embodiment, the electrochemical cell stack ofthe present invention has internal manifolds positioned within theactive area of each electrochemical cell, with each cell comprising anMEA and a partially or fully diffusion bonded bipolar plate assembly.The electrochemical cell stack in this embodiment is preferably apassive water removal cell stack employing hydrophilic porousplate/frame assemblies for water/gas separation, which is suitable forzero gravity operation.

In a second more preferred embodiment, the electrochemical cell stackhas external manifolds (i.e., manifolds positioned outside the activearea of each cell), which communicate with the active area of eachelectrochemical cell, with each cell comprising an MEA and a partiallyor fully diffusion bonded bipolar plate assembly.

Also provided by way of the present invention is a method for passivewater removal from an electrochemical cell or cell stack, the methodcomprising:

providing one electrochemical cell or an arranged group or stack ofcells, as described above, wherein each cell includes an MEA having ananode side and an opposing cathode side, open structures (e.g.,screen/frame assemblies) located on opposing sides of the MEA, ahydrophilic porous plate or porous plate/frame assembly adjacent to andin intimate contact with the open structure located on the cathode sideof the MEA, and a water collection chamber located on an opposing sideof the hydrophilic porous plate or porous plate/frame assembly; and

maintaining the open structure located on the cathode side of the MEA ata pressure greater than the pressure in the water collection chamber inthe cell(s) during operation of the electrochemical cell or cell stack.

Other features and advantages of the invention will be apparent to oneof ordinary skill from the following detailed description andaccompanying drawings.

Unless otherwise defined, all technical and scientific terms used hereinhave the same meaning as commonly understood by one of ordinary skill inthe art to which this invention belongs. All publications, patentapplications, patents and other references mentioned herein areincorporated by reference in their entirety. In case of conflict, thepresent specification, including definitions, will control. In addition,the materials, methods, and examples are illustrative only and notintended to be limiting.

BRIEF DESCRIPTION OF THE DRAWINGS

Particular features of the disclosed invention are illustrated byreference to the accompanying drawings, in which:

FIG. 1 is a top or plan view of one embodiment of the diffusion bondedlaminar assembly of the present invention in the form of an oxygenscreen/frame assembly, while FIG. 1A is an enlarged view of one portionof this assembly showing overlying screen patterns in the assembly;

FIGS. 2A and 2B are both similar to FIG. 1A, and show two differentmanifold approaches, an external manifold approach (FIG. 2A) and aninternal manifold approach (FIG. 2B);

FIG. 3 is an exploded side view of one embodiment of the passive waterremoval (PWR) Proton Exchange Membrane (PEM) H₂/O₂ fuel cell of thepresent invention;

FIG. 4 is a diagrammatic depiction of water removal in a passive waterremoval cell (shown in cross-section) as the reaction proceeds withinthe cell;

FIG. 5 is cross-sectional side view of another embodiment of theelectrochemical cell of the present invention in the form of a PWR, PEMH₂/O₂ fuel cell that utilizes sintered metallic porous membranes forwater/gas separation; and

FIG. 6 is cross-sectional side view of yet another embodiment of theelectrochemical cell of the present invention in the form of a PWR, PEMH₂/O₂ fuel cell that utilizes polymeric porous membranes for water/gasseparation.

BEST MODE FOR CARRYING OUT THE INVENTION

The diffusion bonded electrochemical cell of the present invention is alight weight cell that may be used as a fuel cell, regenerative fuelcell, electrolysis cell, and the like. This novel cell structuresimplifies construction and operation of these cells. Moreover, the highdegree of ruggedness, reliability, efficiency and attitudeinsensitiveness demonstrated by the inventive electrochemical cellsrender these cells suitable for use in applications such as underseapower systems for diving or subsea oil exploration, propulsion or dataacquisition, commercial space power systems, remote backup powersystems, long endurance aircraft, and regenerative fuel cells.

As noted above, the inventive electrochemical cell demonstratesexcellent conductivity and improved resistance to delamination andcomprises two or more diffusion bonded layers.

Referring now to FIG. 1 in detail, reference numeral 10 has been used togenerally designate a diffusion bonded laminar assembly in the form ofan oxygen screen/frame assembly or screen flowfield part used in oneembodiment of the electrochemical cell of the present invention. Thediffusion bonded oxygen screen/frame assembly or screen flowfield part10 is a laminar assembly of four square-shaped layers or components 12,with each component 12 including a central portion 14 having a multitudeof fluid-flow spaces 18 and a frame portion 18 integral with andcircumferentially surrounding the central portion 14.

The oxygen screen/frame assembly or screen flowfield part shown in FIG.1 was made by etching four layers of grade 316L stainless steel withscreen and frame patterns. Each layer had a thickness of 0.004″. Twolayers measured: 0.125″ long way dimension (LWD); 0.055″-0.062″ shortway dimension (SWD); and 0.012″ strand width, while the other two layersmeasured: 0.077′ LWD: 0.038″-0.043″ SWD: and 0.007″ strand width. Thescreen patterns in each set of two layers were orientated perpendicularto one another, and one set stacked on top of the other set such thateach overlying layer in the resulting assembly alternated in screenorientation (see FIG. 1A). The stacked two-layer sets were thendiffusion bonded into a final assembly. No frame-screen transitionregularities were detected in the final assembly. Moreover, the assemblydemonstrated a high degree of flatness and had excellent conductivityacross the screen boundaries.

Two different manifold approaches, which are both suitable for use withthe present invention, are shown in FIGS. 2A and 2B. In FIG. 2A,manifold 20 is positioned external to the active area (i.e. centralportion 14) of the assembly or part, communicating with the active areavia continuous flow channels 22. In FIG. 2B, manifold 20 is positionedover central portion 14, communicating directly with the active area ofthe assembly.

Diffusion bonding is basically a welding process by which a jointbetween similar or dissimilar metals, alloys, or nonmetals is formedwithout the use of adhesives. The process involves pressing twomaterials together (typically in a vacuum) at a specific pressure andtemperature for a particular holding time. Suitable pressures,temperatures, and holding times are well known to those skilled in thediffusion bonding art. Temperatures are typically set at 50-90% of themelting temperature of the most fusible material being bonded.Increasing the temperature aids in the interdiffusion of atoms acrossthe face of the joint. During the diffusion bonding process, holdingtimes are minimized.

Diffusion bonding simplifies cell and stack construction where thisprocess naturally lends itself to automation and thus lower cost.Moreover, diffusion bonding does not produce harmful gases, ultravioletradiation, metal spatter or fine dusts, nor does it require expensivesolders, special grades of wires or electrodes, fluxes or shieldinggases.

Where boundaries between layers disappear during the diffusion bondingprocess, excellent conductivity and resistance to delamination isensured.

The electrochemical cell of the present invention may be manufacturedusing polymer, carbon, graphite, ceramic, composite, or metal basedmaterials, with assembled stacks using either edge current collection ora bipolar current design.

As noted above, the cathode/electrolyte/anode assembly (i.e.,membrane-electrode-assembly or MEA) in the electrochemical cell of thepresent invention has a first flow field in fluid communication with thecathode and a second flow field in fluid communication with the anode.These flow fields (i.e., open structures), which are made up of screenpacks or channeled plates, facilitate fluid movement to and from the MEAand provide mechanical support for the MEA.

In one contemplated embodiment, bipolar plate assemblies are positionedon either side of the MEA and are each made up of an oxygen screen/frameassembly, a porous plate/frame assembly, a water chamber, a firstseparator or divider plate, a coolant chamber, a second separator ordivider plate, and a hydrogen chamber. As will be readily appreciated bythose skilled in the art, the water chamber may also be used as acoolant chamber in which case the bipolar plate assemblies could bestreamlined to only include an oxygen screen/frame assembly, a porousplate/frame assembly, water/coolant chamber, a divider plate, and ahydrogen chamber.

For porous plate/frame assemblies made up of sintered metallic porousmembranes, the bipolar plate assembly may be fully diffusion bonded,while for porous plate/frame assemblies made up of polymeric porousmembranes, the bipolar plate assembly may be partially diffusion bonded.In particular, the polymeric porous membrane would be positioned betweena first diffusion bonded plate assembly (e.g., oxygen screen/frameassembly) and a second diffusion bonded plate assembly (e.g., waterchamber//first divider plate//coolant chamber//second dividerplate//hydrogen chamber).

Referring now to FIG. 3, reference numeral 24 has been used to generallydesignate a preferred embodiment of the electrochemical cell of thepresent invention. In this first preferred embodiment, electrochemicalcell 24 is a passive water removal (PWR) PEM H₂/O₂ fuel cell employingthe diffusion bonded oxygen screen/frame assembly or screen flowfieldpart shown in FIG. 1. The PWR PEM H₂/O₂ fuel cell 24 has internalmanifolds (not shown) positioned within the active area of the cell thatare able to provide communication from cell to cell. Fuel cell 24comprises: a first separator or divider plate 26; a water chamber orflowfield 28; a hydrophilic porous plate 30; a diffusion bonded oxygenscreen/frame assembly 32; an MEA 34; a hydrogen flowfield 36; and asecond separator or divider plate 38.

Upon testing, fuel cell 24 successfully performed the function of watermanagement, gas admission, and gas distribution across the face of thecathode of the fuel cell.

By way of explanation, many PEM fuel cells remove product water byentraining this water in a flow of excess air or oxygen through thecathode side of the cell or by evaporation of water into a circulatinggas stream. This excess flow delivers cell product water out of the cellwhere it is then separated or vented. While this is functional, itrequires the introduction of additional system components that can addweight and complexity to a fuel cell system.

In the embodiment shown in FIG. 3, product water is removed directlyfrom the cell with no flow circulation of reactants. The cell offerssuperior performance, lighter weight, and more durability.

The technique of passive water removal in a fuel cell, as well as thepore size and shape requirements for the hydrophilic porous plate, istaught in U.S. Pat. No. 4,729,932 to McElroy, which is incorporatedherein by reference. By way of further explanation, and as best shown inFIG. 4, as the fuel cell reaction within a passive water removal cellproceeds, liquid water is formed at the cathode side of the MEA. Morespecifically, water molecules group together forming small droplets thatgrow to a radius approximately equal to the mesh size of the screen mesh(e.g., oxygen screen/frame assembly) located between the MEA and theporous plate. The water droplets fill one or more openings in the screenmesh, and then grow to span the distance between the MEA and the porousplate. Due to the properties of the porous plate, the water droplets arethen quickly transported across the plate to the water chamber.

As is known to those skilled in fluid mechanics, the Bond number (Bo)represents the ratio of body forces (typically gravitational) to surfacetension forces. If the Bo is much greater than 1, gravity dominates, andif the Bo is much less than 1, surface tension/energy dominates. Thepresent inventors have determined that the Bo for the preferred PWR cell24 is less than 0.05, which confirmed that surface tension effectspredominated in cell 24, thereby confirming that the PWR process issuitable for zero gravity operation. The Bo was calculated in accordancewith the following equation:

${Bo} = \frac{\rho \; a\; L^{2}}{\gamma}$

where ρ is water density, a is the acceleration associated with the bodyforce, typically gravity, L is the characteristic length scale, and γ isthe surface tension of the interface.

The method for passive water removal from an electrochemical cell orstack of the present invention may be described as:

providing one electrochemical cell or an arranged group or stack ofcells, as described above, wherein each cell includes an MEA having ananode side and an opposing cathode side, open structures (e.g., screenmesh or screen/frame assemblies) located on opposing sides of the MEA, ahydrophilic porous plate adjacent to and in intimate contact with theopen structure located on the cathode side of the MEA, and a watercollection chamber located on an opposing side of the hydrophilic porousplate; and

maintaining the open structure located on the cathode side of the MEA ata pressure greater than the pressure in the water collection chamber inthe cell(s) during operation of the electrochemical cell or cell stack.

Referring now to FIG. 5, reference numeral 40 has been used to generallydesignate a section from a first preferred embodiment of theelectrochemical cell stack of the present invention. In this firstpreferred embodiment, electrochemical cell stack 40 is a PWR PEM H₂/O₂fuel cell stack that utilizes hydrophilic metallic porous membranes forwater/gas separation. This cell stack, which has either internal orexternal manifolds (not shown), employs fully diffusion bonded repeatingbipolar plates 42 a, 42 b positioned on either side of each MEA 44. Thebipolar plate assemblies 42 a, 42 b are each made up of an oxygenscreen/frame assembly 46 a, 46 b, a sintered metallic porous membrane orporous plate/frame assembly 48 a, 48 b, a water chamber 50 a, 50 b, afirst separator or divider plate 52 a, 52 b, a coolant chamber 54 a, 54b, a second separator or divider plate 56 a, 56 b, and a hydrogenchamber 58 a, 58 b.

Preferred power outputs of fuel cell stack 40, as shown in FIG. 5, rangefrom about 1 to about 20 kW nominal @200 ma/cm².

During operation of PWR fuel cell stack 40, the water chamber side ofthe sintered metallic porous membrane or porous plate/frame assembly 48a, 48 b in each cell is maintained at a pressure below that of theoxygen screen/frame assembly 46 a, 46 b side of the sintered metallicporous membrane or porous plate/frame assembly 48 a, 48 b.

FIG. 6 depicts a second preferred embodiment of the electrochemical cellstack of the present invention. Reference numeral 60 has been used togenerally designate this variation on design. In this second preferredembodiment, electrochemical cell stack 60 is a PWR PEM H₂/O₂ fuel cellstack that utilizes hydrophilic polymeric porous membranes for water/gasseparation. This cell stack also has either internal or externalmanifolds (not shown) and has partially diffusion bonded repeatingbipolar plates 62 a, 62 b positioned on either side of each MEA 64 a, 64b. The bipolar plate assemblies 62 a, 62 b are each made up of apolymeric water-gas porous membrane 66 a, 66 b that is supported oneither side by the following assemblies: a diffusion bonded oxygenscreen/frame assembly 68 a, 68 b, and a diffusion bonded assembly madeup of a water chamber 70 a, 70 b, a first separator or divider plate 72a, 72 b, a coolant chamber 74 a, 74 b, a second separator or dividerplate 76 a, 76 b, and a hydrogen chamber 78 a, 78 b.

The polymeric water-gas porous membrane 66 a, 66 b, of electrochemicalcell stack 60 may or may not be electrically conductive. If electricallyconductive, cell stack 60 may utilize current flow through the center ofthe cell. If not electrically conductive, current may flow external toeach cell using an edge conduction approach, or active area conductionpathways.

One noteworthy advantage of fuel cell stack 60 is its light weightdesign, made possible by the very thin cells prepared using thisdiffusion bonding process.

During operation of PWR fuel cell stack 60, the differential pressureacross the polymeric water-gas porous membrane 66 a, 66 b in each cellis maintained such that the pressure of the water chamber 70 a, 70 bside of the polymeric water-gas porous membrane 66 a, 66 b is below thatof the oxygen screen/frame assembly 68 a, 68 b side of the polymericwater-gas porous membrane 66 a, 66 b.

While various embodiments of the inventive electrochemical cell andarranged groups or stacks of such electrochemical cells have beendescribed herein, it should be understood that they have been presentedby way of example only, and not limitation. Thus, the breadth and scopeof the present invention should not be limited by any of the exemplaryembodiments.

1. An electrochemical cell that comprises two or more diffusion bondedlayers.
 2. The electrochemical cell of claim 1, which comprises a porousplate/frame assembly for water/gas separation, wherein the porousplate/frame assembly utilizes a membrane selected from the group ofpolymeric porous membranes and metallic porous membranes.
 3. Theelectrochemical cell of claim 2, which comprises a porous plate/frameassembly utilizing a polymeric porous membrane and at least onediffusion bonded plate assembly, the porous plate/frame assembly beingsupported on at least one side with the at least one diffusion bondedplate assembly.
 4. The electrochemical cell of claim 3, which comprisesa porous plate/frame assembly utilizing a polymeric porous membrane anda bipolar plate assembly, the bipolar plate assembly made up of a firstdiffusion bonded plate assembly and a second diffusion bonded plateassembly, the porous plate/frame assembly being supported on one sidewith the first diffusion bonded plate assembly and on an opposing sidewith the second diffusion bonded plate assembly.
 5. The electrochemicalcell of claim 4, wherein the first diffusion bonded plate assemblycomprises an oxygen screen/frame assembly.
 6. The electrochemical cellof claim 4, wherein the second diffusion bonded plate assembly comprisesthe following layers in the order specified, (a) a water/coolantchamber, (b) a divider sheet, (c) a hydrogen chamber.
 7. Theelectrochemical cell of claim 4, wherein the second diffusion bondedplate assembly comprises the following layers in the order specified,(a) a water chamber, (b) a first divider sheet, (c) a coolant chamber,(d) a second divider sheet, and (e) a hydrogen chamber.
 8. Theelectrochemical cell of claim 2, which comprises at least one diffusionbonded plate assembly that includes a porous plate/frame assemblyutilizing a metallic porous membrane.
 9. The electrochemical cell ofclaim 8, wherein the at least one diffusion bonded plate assembly is abipolar plate assembly that comprises the following layers in the orderspecified, (a) an oxygen screen/frame assembly, (b) the porousplate/frame assembly, (c) a water/coolant chamber, (d) a divider sheet,and (e) a hydrogen chamber.
 10. The electrochemical cell of claim 8,wherein the at least one diffusion bonded plate assembly is a bipolarplate assembly that comprises the following layers in the orderspecified, (a) an oxygen screen/frame assembly, (b) the porousplate/frame assembly, (c) a water chamber, (d) a first divider sheet,(e) a coolant chamber, (f) a second divider sheet, and (g) a hydrogenchamber.
 11. The electrochemical cell of claim 1, which furthercomprises a hydrophilic porous plate or porous plate/frame assembly. 12.The electrochemical cell of claim 11, which is a passive water removalPEM H₂/O₂ fuel cell.
 13. An arranged group or stack of electrochemicalcells, wherein each electrochemical cell comprises two or more diffusionbonded layers.
 14. The arranged group or stack of electrochemical cellsof claim 13, wherein each electrochemical cell further comprises ahydrophilic porous plate or porous plate/frame assembly.
 15. Thearranged group or stack of electrochemical cells of claim 13, whereineach cell has an active area, the active areas of the cells being inoverlying arrangement, wherein the cell stack has internal manifoldspositioned within the active areas of the cells, the internal manifoldscommunicating directly with these active areas.
 16. The arranged groupor stack of electrochemical cells of claim 15, wherein eachelectrochemical cell further comprises a hydrophilic porous plate orporous plate/frame assembly.
 17. The arranged group or stack ofelectrochemical cells of claim 13, wherein each cell has an active area,the active areas of the cells being in overlying arrangement, whereinthe cell stack has external manifolds which communicate with theseactive areas by way of continuous flow channels located between theexternal manifolds and the active areas.
 18. The arranged group or stackof electrochemical cells of claim 13, wherein each cell comprises aporous plate/frame assembly utilizing a polymeric porous membrane and atleast one diffusion bonded plate assembly, the porous plate/frameassembly being supported on at least one side with the at least onediffusion bonded plate assembly.
 19. The arranged group or stack ofelectrochemical cells of claim 13, wherein each cell comprises at leastone diffusion bonded plate assembly that includes a porous plate/frameassembly utilizing a metallic porous membrane.
 20. A method for passivewater removal from an electrochemical cell or cell stack, the methodcomprising: providing one electrochemical cell or an arranged group orstack of cells, wherein each cell includes a membrane electrode assemblyhaving an anode side and an opposing cathode side, open structureslocated on opposing sides of the membrane electrode assembly, ahydrophilic porous plate or porous plate/frame assembly adjacent to andin intimate contact with the open structure located on the cathode sideof the membrane electrode assembly, and a water collection chamberlocated on an opposing side of the hydrophilic porous plate or porousplate/frame assembly, and maintaining the open structure located on thecathode side of the membrane electrode assembly at a pressure greaterthan the pressure in the water collection chamber in the cell or cellsduring operation of the electrochemical cell or cell stack.